A secondary query loop using $sec_query = new WP_Query() and $sec_query->the_post() affects the global $post variable. The global $post variable is used by template tags by default. wp_reset_postdata() restores the global $post variable to the current post in the main query (contained in the global $wp_query variable as opposed to the $sec_query variable), so that the template tags refer to the main query loop by default again.